The invention relates to a laser system for generating ultra-short light pulses, with a laser resonator which comprises at least one active solid-state oscillator element, preferably a thin disk of a laser-active crystal with a high amplification bandwidth, particularly of a xe2x80x9cquasi-three-levelxe2x80x9d system, such as Yb:YAG, and a means for phase coupling of the laser modes and for dispersion compensation, respectively.
In recent years, a dramatic breakthrough was achieved in the technology of ultra-short pulses. The typical femtosecond-pulse sources of the preceding decade, (gas) ion laser pump sources and (liquid) dye oscillators and amplifiers have given way to solid-state systems, particularly the Ti:sapphire oscillators which are pumped by diode-pumped, frequency-doubled Nd-lasers, and the Ti:sapphire amplifiers, which are pumped by lamp-pumped frequency-doubled Nd-lasers. Another type of femtosecond laser is based on Cr-doped fluoride crystals (LiSGaF, LiSAF), which can be directly diode-pumped at 670 nm and approach the performance of the Ti:sapphire with reference to the pulse period. Other output parameters such as pulse energy and average power have also been notably improved. However, due to the high complexity of the apparatuses for high powersxe2x80x94multi-step architecture of the systemsxe2x80x94and to the high apparatus and control outlay and the high costs associated with this, real economic applications are not yet foreseeable. For the pulse period range under 100 fs, which is rather insignificant for non-scientific applications, a high outlay can be justified, given complex solutions, but for the economically significant pulse period between 100 and 500 fs, there are no suitable alternatives for high average powers over 1 W.
Because of its good and proven thermo-optical properties and low Stokes dislocation, a Yb:YAG crystal is an ideal laser medium, said crystal enabling high average powers and permitting direct pumping on the basis of suitable absorption properties at available high-power diode wavelengths and of a long lifetime of the excited state.
By means of a new concept, that of the disk laser described in U.S. Pat. No. 5,553,088, a power scale ability in the range of ca. 10 W to ca. 1 kW average power has been successfully achieved for high-power systems. The reduced thermal lens effect and the potentially compact architecture allow a compact structure which is suitable for industrial applications, along with an effectivity of over 50%.
The generation of ultra-short pulses with high energies, which is necessary for the processing of material with new properties such as acoustic and thermal shock protection, for example, has not yet been realized with this type of solid-state laser, nor are there any approaches toward achieving this object indicated in the cited patent.
It is thus the object of the present invention to provide a laser system which can emit ultra-short pulses in a power-scalable fashion with very high average powers for industrial applications, and which is also constructed optimally simply and without great outlay.
This object is inventively achieved in that the means for phase coupling is constructed as a passive, non-linear element, and that a prismless means for dispersion compensation is provided. The passive phase coupling via non-linear optical elements enables the simplest generation of ultra-short pulses. A broader range of average powers is achieved specifically with Yb:YAG in the form of a disk laser with at least one laser-active element in the form of a preferably 200 to 400 xcexcm thick disk several millimeters in diameter, this system being suitable for the generation of pulses in the xcexcJ and mJ range (latter with amplifier) with pulse widths of approximately 200 fs. Furthermore, long beam paths, partially in glass, can be avoided by means of the prismless construction of the means for dispersion compensation; the adjustment stability is better maintained; and the post-calibration is less complicated. Overall, prismless means are more compact, less maintenance-intensive and more economical.
For laser systems of lower power (a few 100 mW) and on a different basis, a passive mode-coupling method was realized by means of a Kerr lens device (see EP-0 492 994 A2); yet there are no indications of the application thereof in high-power laser systems with ultra-short pulses. Rather, further progress was made with semiconductor-based saturable absorbers (SESAMs), which, however, have proven to be too short-lived for high-power laser systems with respect to their utility at powers over a few 100 mW. The non-linear optical methods are preferable, since they can be power-scaled by means of corresponding focusing of the beam and are not based on the direct absorption of radiation.
According to another feature of the invention, it is provided that the means for phase coupling is constructed as a Kerr lens phase coupling device, preferably with a soft diaphragm. An APM device or a non-linear mirror arrangement with non-linearity of the 2nd order can also potentially be provided. These variants permit a power-scalable generation of ultra-short laser pulses with high energies, without the means for phase coupling being damaged by the high energies. Powers of well over 10 W can thus be achieved in the resonator, while the safe power range in the most common techniques using semiconductor-based, saturable absorbers is only between 0.01 and about 1 W.
For the achievement of a construction which is compact and simplified for industrial applications, it is preferably provided that at least one laser-active element is simultaneously a non-linear element of the means for phase coupling, preferably of the Kerr lens phase coupling device. With this construction, which is designed for the standard X-shaped resonator shape, a significant simplification of the structure is possible, along with its reduction in size.
Given very thin active laser media, specifically, in order to compensate the potentially lacking self-focusing and to achieve sufficient non-linearity for the phase coupling, according to another feature of the invention, an additional focusing zone of the laser light is established inside the laser resonator, and a self-focusing, transparent optical material is arranged in this focusing zone.
According to a preferred embodiment, a system of dielectric dispersion compensation mirrors, preferably Gires-Turnois interferometer mirrors, is provided as a prismless arrangement for dispersion compensation. Based on the typical bandwidth of 20 to 30 nm, these mirrors can process pulses to down to 40 fs; they demonstrate particularly low losses under 0.1% per reflection and high dispersion from 100 to 150 fs2 per reflection. The power processing properties of the Gires-Turnois mirror are also excellent, and so they are most suitable specifically for the maintenance, or respectively, guaranteeing of the power scaleability even given large values.
According to an advantageous development of the invention, it is provided that a regenerative amplifier is connected downstream to the output coupler of the laser resonator. This type of amplification is suitable specifically for laser-active media with a low amplification factor, and the single pulses of the oscillator, which do not comprise more than 1 to 2 xcexcJ, even given average output powers of up to 100 W, can thus be amplified in a relatively simple manner to into the mJ range, given repetition rates of about 10 kHz.
It is advantageously provided that the regenerative amplifier comprises a laser-active solid, preferably a thin disk of a laser-active crystal with a high amplification bandwidth, particularly of a xe2x80x9cquasi-three-levelxe2x80x9d system such as Yb:YAG. This guarantees advantages such as identical technology, long lifetime and power scale ability not only for the primary laser oscillator but also for the amplifier.
In order to avoid a damaging or compromising of the amplifier given high amplifications, the regenerative amplifier inventively comprises means at the input side for the dispersive elongation of the laser pulse and means at the output side for the recompression of the laser pulse. This enables a safe raising of the energy of the single pulse to up to 10 mJ, which corresponds to an optical peak power in the range of 10 GW/cm2.
But a raising of the single pulse energies can also be achieved in that the laser resonator contains a cavity dumping circuit or a Q-circuit. Repetition rates in the range of 1 MHz and pulse energies of about 10 xcexcJ can be achieved by cavity dumping, for example.
Said circuits can be realized in a simple and proven manner as Bragg or Pockels cells or by an acousto-optical modulator.